Method for Manufacturing Cast Parts

ABSTRACT

The invention relates to a method for the production of moulded parts made of light-metal alloys, particularly using a diecasting method, wherein moulded parts having a reduced weight are produced. According to the invention, the mould cavity ( 2 ) is filled with a metal melt, a peripheral layer is hardened, a gas is introduced into the remaining melt and the melt is displaced from the moulding cavity ( 2 ), forming a hollow area ( 5 ).

The invention relates to a method for manufacturing cast parts out of light-metal alloys, in particular via pressure diecasting, according to the preamble of the claim.

The use of gases during the manufacture of metal cast parts is known in the art. For example, DE-A-3240242 describes a method for pressure diecasting with the simultaneous exposure to gas counter-pressure. The characteristic feature is that the casting mold cavity and a horizontal pressure chamber are separated before the casting process tarts, after which a gas pressure is generated in the casting mold cavity, and the melt is simultaneously poured into the horizontal pressure chamber, while the cavity and horizontal pressure chamber space are connected as casting begins. The gas pressure is independent of melt flow.

DE-OS 2823173 discloses a method for casting metal in a rotating die set, in which the surface of the metal melt is simultaneously protected by liquefied inert gas during the casting process. The gas is poured both on the melt surface and inside the rotating mold. A first metal is cast, pressed against the mold wall via centrifugation, and solidifies with the formation of a first, outer layer, after which the supply of a liquid gas is initiated. In like manner, a second layer is subsequently cast. The additionally supplied liquid gas fills the inner cavity of the mold until a finished turned piece is obtained.

In a method for casting hollow items via slush casting according to DE-A-3403186, air is introduced inside the casting jacket to facilitate the discharge of remaining melt. The air is supplied through an opening created in the casting jacket or similarly to the melt.

The object of the invention is to develop a method for manufacturing cast parts while avoiding the disadvantages of prior art, in particular for cast parts made out of light-metal alloys and according to a pressure diecasting process.

The object is achieved with the features of the patent claim.

The invention here proceeds from known injection-molding methods for plastic parts, in which cavities with a uniform interior gas pressure in the cavity are generated by injecting a gas. Because the thermophysical properties relative to thermoplastics differ greatly during the solidification of metal melts, these methods cannot be simply applied during the pressure diecasting of metal cast parts. Not only are the operating temperatures higher, solidification proceeds much faster, so that a casting cycle proceeds significantly faster.

The method involving gas injection can be implemented via

-   -   Partial filling (residual melt is again aspirated into the         casting chamber after gas injection)     -   Shrinkage compensation     -   Core pulling system     -   Overflow cavity

Executing the process with an overflow cavity is preferred for metal melts (low viscosities), and is characterized by the following steps:

-   a) A cavity is filled with melt, and a secondary cavity (overflow     cavity) of the mold is sealed; -   b) After solidification of an edge layer, gas is introduced into the     remaining melt through an injector, and access to the secondary     cavity is opened; -   c) The remaining melt gets into the secondary cavity, at least one     hollow space remains in the cast part; -   d) The cavities are ventilated, the secondary cavity section is     taken out partially and removed.

While nitrogen is preferably used as the gas, any other inert gas is also suitable. To achieve surfaces with the smoothest possible walls (even on the inside) for the cast part, alloys with a short solidification interval are preferably used, e.g., zinc, aluminum or magnesium alloys. A local post-pressure can ward off sink marks and/or warping.

The solidification front of the metal solidifies during exposure to gas pressure. This results in the formation of smooth, solidified walls. This aspect is if utmost importance for the subsequent use of hollow cast parts as a line that carries media, whether it be gas or liquid, since the flow resistance is greatly diminished by how smooth the walls are on the inner surface.

Possible outcomes include a reduction in cycle time for thick-walled parts, improved stability, firing force owing to a uniform pressure, and minimized warping resulting from reduced material accumulations.

Lightweight cast parts can be obtained, e.g., door handles or oil filter casings.

The invention will be described in greater detail below in an exemplary embodiment based on a drawing, but without being limited to this specific embodiment. The drawing shows the procedural sequence in only one figure.

The metal melt located in the casting chamber 1 of a pressure diecasting machine is pressed into a mold cavity 2 by means of a casting piston by way of an optional valve 6 (the method can also be implemented without using a valve 6). Another valve 7 for a secondary cavity 4 is closed. As soon as an edge layer of the melt has solidified on the inner wall of the mold cavity 2, a gas injector 3 introduces nitrogen into the mold cavity 2, or in the remaining melt, and the valve 7 of the secondary cavity 4 is opened at the same time. The valve 6 is now closed. The remaining quantity of metal melt is forced by the gas pressure into the secondary cavity 4. At least one hollow space 5 is formed in the mold cavity 2 or cast part.

The cavities, in particular the hollow space 5, are subsequently ventilated via the gas injector 3, after which the cast part can be removed from the mold and further processed. 

1. A method for manufacturing cast parts made of light-metal alloys, in particular via pressure diecasting, comprising introducing metal melt into a mold cavity, wherein, after an edge layer has solidified on the inner wall of the mold cavity, the remaining metal melt is at least partially forced out of the mold cavity by introducing gas.
 2. The method according to claim 1, wherein at least one hollow space is created in the cast part by introducing gas and forcing out the melt.
 3. The method according to claim 1 wherein the remaining metal melt is forced into a secondary cavity by the introduced gas.
 4. The method according to one of claims 1 wherein the gas is nitrogen.
 5. A device for manufacturing cast parts while implementing the method according to at least one of claims 1 wherein a mold exhibits a gas injector and a secondary cavity.
 6. The device according to claim 5, wherein the secondary cavity is connected with the mold cavity by a valve. 